PROJECT SUMMARY/ABSTRACT PROJECT I: LEARNING DIFFICULTIES IN READING, WRITING, AND MATHEMATICS AND THEIR CO-OCCURRENCE WITH ADHD: ETIOLOGY, NEUROPSYCHOLOGY, AND FUNCTIONAL OUTCOMES The overall goal of Project I is to examine the relations among all dimensions of learning disabilities and ADHD at the phenotypic, etiological, and developmental levels of analysis and to begin to test these relations in historically understudied and underserved populations. The first primary objective of Project I is to conduct an etiologically-informative study of all aspects of learning disabilities in reading (RD), math (MD), and writing (WD), along with the etiology of covariance among these dimensions of academic difficulties and symptoms of attention-deficit/hyperactivity disorder (ADHD) and the related but distinct dimension of attentional functioning called sluggish cognitive tempo. Tests of basic and higher-order dimensions of reading (word reading, reading fluency, and reading comprehension), math (math calculations, math fluency, and math reasoning), and writing (handwriting, spelling, grammar, and written content) will be administered to a sample of 250 twin pairs between 8 and 12 years of age in which at least one twin has a school history of learning and/or attentional difficulties, along with a comparison sample of 100 twin pairs without learning or attentional difficulties. These data will then be combined with twin pairs tested during the current and previous funding period of the CLDRC to provide sufficient statistical power to test competing models of the phenotypic and etiological structure of all aspects of learning disabilities (LDs). Results of these models will provide a foundation for synergistic collaborative analyses that examine the relations between the latent measures of learning difficulties that are identified and measures of processing speed and executive functions (Project I and II), structural and functional neuroimaging phenotypes (Project III), and molecular genetic risk factors (Project IV). In addition, Project I will conduct include two longitudinal followup studies. In the first study approximately 180 pairs of twins from the CLDRC and an independent sample of 100 bilingual (English and Spanish) Hispanic adolescents with and without RD will complete a streamlined version of the Project I and Project II test batteries approximately five years after they were first tested. The second longitudinal study will administer an online battery of outcome measures to over 1,200 pairs of young adult twins who previously participated in the CLDRC or one of our ongoing longitudinal twin studies of unselected samples in Colorado or Ohio, providing a cost-effective assessment of important longer-term developmental outcomes of LDs.